HERMS Temperature Control Help

Bear with me as I give a little background before my question...

My first brew with my new HERMS setup did not go as well as I had hoped. I chose to control the temperature at the exit of the HLT HERMS coil (Mash.pv in the diagram). I also have a temperature probe in the bottom of my mash tun (Mash.pv1) and another at the exit of the wort circulation pump (Pump_T).

As you can see in the attached chart I have some control tuning opportunities but that was expected. The target mashing temperature for this recipe was 155F. I tried a cascade control scheme where Mash.pv was the input to one PID whose output set the setpoint for the HLT temperature PID. I should have just controlled it manually the first time to gain experience.

The question is should I control Mash.pv or Mash.pv1? Which is a better indicator of the temperature of the wort and grain? What is a reasonable tolerance for the mash temperature? How close is close enough?

There seem to be a lot of opinions on this, although from what I've seen they can both work just fine assuming the PID is tuned correctly. By both I mean HERMS output (your Mash.pv) or the HLT directly (HLT_T.pv). Monitoring the mash directly (Mash.pv1) can give you problems unless your PID can also monitor and set a max temp of returning wort. I'm not aware of a cheap commercially available PID capable of this.

I personally monitor just the HLT, but I'm not saying it's necessarily the best for every system. The biggest reason for me is simplicity, although it ended up being even more accurate than I thought. There were no compromises I had expected. I have a 2-vessel system and this allows me to have just 1 probe in the whole brewery. Since my HLT doubles as the BK, I would still need another probe/element/PID/complexity of panel design. In the end I have just 2 vessels, 1 PID, 1 probe, 1 element, 1 stainless, coil/chiller, etc. The goal for me was a simple elegant design without resorting to lifting a bag out of cloudy chunky wort.

Obviously a separate probe where the mash exits the HERMS coil will, in theory, best tell you the temperature of the returning wort, but it's mostly theory w/out the reality in the scope of a home brewery.

On my first test, I set the HLT to 150. The HLT stayed as 150. I never saw 149.9 or 150.1. This is to be expected with a properly tuned PID.

The return was 149.9. I never saw 149.8 or 150.0. I don't about you, but I can live with a 0.1*F correction with almost no fluctuation. It was well worth eliminating a bunch of unnecessary complexity. The rest of the mash (un-insulated) settled about 0.1 below that, which would be the same either way.

My probe is where the wort returns to the MLT. I usually have 2 degrees less than what i have the pid set at. I can reliably compensate for that bc its consistent. Every system has it's quirks.

I control the temperature of the HLT via PID and RTD at the HLT, but monitor the mash return and mash itself with simple read outs. I've found that holding the HLT temp from 1.5 to 2 deg F above the target mash temp is about right, and that with 40' of 3/8 ss coil, the wort return temp is the same as the HLT temp.
I use an insulated keggle, and only recirculate when mash temp drops 1 deg F or more.